In an SDMA method, users are distinguished from one another according to their location. This is achieved by adjusting the beams of receiver antennas at the base station to the desired directions according to the location of mobile stations. For this purpose, adaptive antenna arrays, or phased antennas, are used together with signal processing, which enables monitoring the mobile stations.
In a CDMA method, which is often used in the SDMA system, a narrowband data signal of the user is multiplied by a spreading code of a considerably broader band than the data signal and spread over a relatively broad band. Bandwidths of more than 1 MHz are used. Multiplying the data signal spreads it over the whole band used. Simultaneous users share the same frequency band for transmission. A separate spreading code is used in each connection between a base station and a mobile station, and the signals of the different users can be distinguished in the receivers from one another on the basis of each user's spreading code.
In a base station of the SDMA radio system in particular, signal reception and transmission are performed using an antenna array comprising several antenna elements. Each antenna element is typically connected to one transceiver unit. The signals transmitted and received in a common baseband unit are phased in relation to each other in order to have the antenna array produce a radiation pattern of a desired form. A typical antenna pattern consists of a narrow main beam and several side beams. The direction and width of the main beam can be adjusted by phasing the radio frequency signal of each element. In practice, the phasing is performed by multiplying the digitized baseband antenna signal of each antenna element by complex coefficients shaping the antenna pattern.
A good antenna pattern is produced e.g., when the phase and strength of each antenna signal are identical. This kind of situation is, however, usually not possible because the nonidealities and environment of electronic components have an effect on the transceiver unit. This causes relative differences in phase and strength between the antenna signals, which cause side beams and degradation of the main beam. This in turn, leads to interference harmful to the base station and the whole radio system and to reduced radio coverage.